This invention relates to pressure transducers and more particularly to differential pressure capacitive transducers and methods for making same, which transducers are low cost yet capable of measuring very small pressure differentials with extreme accuracy and providing, in conjunction with associated electronic circuitry, an output signal that is highly linear.
Differential pressure capacitive transducers generally comprise a disk-shaped sensing diaphragm having a peripheral margin securely mounted between a pair of support members or reference plates. The surface of each reference plate facing the diaphragm has a depression defining a fluid cavity. The depressed surfaces of the reference plates have conductive coatings which, in conjunction with metal diaphragm surfaces, form the electrodes of a pair of variable capacitors. Differences between the pressure of the fluid on opposite sides of the diaphragm cause the diaphragm to deflect, the magnitude of the deflection being a function of the differential pressure across the diaphragm. The deflection in turn is manifested by changes in the capacitances of the two variable capacitors; those capacitances are measured differentially by appropriate electronic circuitry providing a transducer output signal.
Differential pressure capacitive transducers of the above-described kind are known in which the depressions defining the fluid cavities have concave contours. Examples of such transducers are disclosed by the following U.S. patents:
U.S. Pat. No. 2,667,786 (Spaulding) PA1 U.S. Pat. No. 2,999,385 (Wolfe) PA1 U.S. Pat. No. 2,999,386 (Wolfe) PA1 U.S. Pat. No. 3,232,114 (Ferran) PA1 U.S. Pat. No. 3,557,621 (Ferran) PA1 U.S. Pat. No. 3,618,390 (Frick) PA1 U.S. Pat. No. 3,800,413 (Frick)
As taught by Spaulding in U.S. Pat. No. 2,667,786, one advantage of using a concave fixed surface (as opposed, for example, to a flat-bottomed surface) is that greater sensitivity is provided because the average spacing between the diaphragm and the reference surfaces is considerably less. Another advantage of a concave contour, as disclosed in U.S. Pat. No. 2,999,385 (Wolfe), is that it can be shaped to conform to the curvature of the deflected diaphragm and thereby serve as a support surface against which the diaphragm can uniformly bottom out under overpressure conditions.
Despite the advantages of such contoured surfaces, prior art devices evidence several drawbacks. The principal disadvantage is that the cavities must be formed by grinding the reference plates--typically made of an insulating material such as glass or ceramic--to the desired contour. Since the natural deflection curve of the sensing diaphragm is a complex function, the production of the desired shape with the required degree of precision introduces a costly machining operation. Thus, in the manufacture of prior art transducers a composite is struck: machining costs are reduced somewhat by settling for simpler contours. Thus, U.S. Pat. No. 3,618,390 discloses grinding a spherical surface into the glass while in U.S. Pat. No. 3,557,621 a "conic" surface is recommended. However, while simpler, those contours at best only approximate the natural deflection curve of the diaphragm. The results are that the diaphragm does not bear uniformly against the cavity surface during overpressure conditions, thereby increasing the possibility of fatique and failure of the diaphragm, and that the linearity of the transducer output is reduced.
Thus, one object of the present invention is to provide a method of making a differential pressure capacitive transducer in which the fluid cavities adjacent the diaphragm can be precisely contoured to the natural deflection curve of the diaphragm in an exceedingly simple and inexpensive manner.
Another drawback of the existing devices is that as soon as any part of the sensing diaphragm bottoms out, the capacitor electrodes on that side of the transducer are shorted out resulting in an erratic output signal.
Thus, another object of the present invention is to provide a capacitive pressure transducer in which the sensing diaphragm can bottom out against a conforming surface of the reference plate during overpressure conditions without shorting the capacitor electrodes.
The prior art, as exemplified by the above-identified Frick patents, also discloses the use of additional diaphragms that isolate the sensing diaphragm from the fluids whose differential pressure is to be measured. Those fluids are coupled to the sensing diaphragm through liquid, typically in the form of an inert oil, that completely fills the spaces bounded by the isolation diaphragm on either side of the sensing diaphragm. The isolation diaphragms are corrugated so as to facilitate flexing and, as shown by the Frick patents, the outer surfaces of the reference plates are correspondingly shaped to conform to the corrugations to permit the isolation diaphragms to be disposed very close to the outer surfaces to reduce the volume of oil and thereby minimize the effects of temperature variations. To achieve the desired close proximity of the isolation diaphragms and the reference plates while avoiding contact between them, highly precise machining techniques are used to conform the shape of the outer surfaces of the reference plates to that of the corrugated diaphragm, again adding considerable manufacturing costs.
It is therefore another object of the present invention to provide a simple, low cost method of making the reference plates and the isolation diaphragms while assuring that the surface shapes of the plate and diaphragm match precisely.